Here are a few tips for troubleshooting generator failures.
1. Update all of plugins or themes
Shifter automatically updates its WordPress to the latest but doesn't do with plugins or themes, so you should manually do it. The older plugins or themes might cause the failure of artifact creation or missing or corrupted posts or pages on the artifact.
Never forge update plugins and themes to the latest after logging in to WordPress.
2. Disable and Remove WordPress Caching Plugin
Are you running Caching plugin for WordPress? Using caching Plugins can be helpful on traditional hosting platforms but they aren't needed here. Shifter requests each page of your site to generate static versions and caching plugins can conflict with that process.
Some common WordPress Caching Plugins to disable or remove while using on Shifter are the following.
SNS Count Cache
WP Super Cache
WP Fastest Cache
W3 Total Cache
3. Remove unused themes and plugins
During the generating process Shifter will index, archive, and copy all the files located within your WordPress install. Themes and plugins installed but not active also get copied. Those plugins or themes could be creating a timeout during the generating process.
Check to see if there are any plugins or themes that you can remove completely.